This invention relates to power drive systems and more particularly to a torque limiting drive for stern drive watercraft.
Under ideal operating conditions, a stern drive is only subjected to an engine's rated torque for a given revolutions per minute (“RPM”). If the drive is properly matched to the engine's output, the drive should provide dependable service for its intended life. However, boats or watercraft are frequently operated in waters that cause the propeller to become aerated and then re-submerged due to wave action. By throttling, the operator attempts to control the engine from over-revving and minimize strain on the stern drive by attempting to match the revolutions of the propeller with the speed of the boat as it reenters the water.
Even with significant practice, it is unreasonable to believe that the operator can perfectly match propeller revolutions and boat speed every time. Furthermore, even “perfect” throttling cannot completely compensate for the rotational inertia of the engine/flywheel that occurs when the free-spinning propeller reenters the water and is shock loaded. Basically, a spinning propeller that is suddenly submerged will experience a large force or spike of enormous drag until the boat speed and RPMs are balanced.
Generally the engine's crankshaft, flywheel and coupler weigh more than 100 pounds and are rotating 30% to 50% faster than the propeller depending on the gear ratio of the drive. The rotational inertia of these components is the source of the shock loading on the stern drive. This excessive loading generates torque spikes in the stern drive and quickly leads to damaged gears, twisted or broken shafts and damaged housings. For most recreational boaters, stern drive repairs account for the majority of their unscheduled maintenance costs.
Applicant's invention is a method and device designed to manage the shock-induced loading of the gear sets and shafts of a marine stern drive to reduce or eliminate damage from overloading or impact with a submerged object. The invention limits the maximum torque experienced by the drive with minimal or no impact or adverse effects under normal operation. In operation, the invention operates automatically, requires no additional maintenance and is completely transparent to the boat operator.
The torque limiting clutch system replaces the existing coupler that connects the engine's flywheel to the stern drive's input shaft or driveline in staggered installations. The stock bell housing is utilized and no other modifications are required.
The invention operates in the stern drive system of a watercraft by using a torque-limiting clutch designed to momentarily slip when the torque exceeds a preset value. Since the breakaway torque rating is higher than the engine's output there will be no impact or slippage of the clutch under normal operating conditions. When the torque exceeds the pre-set value, such as when an airborne propeller re-enters the water, the shock or load to the drive spikes and the clutch slips until the propeller's revolutions synchronize with the engine, reducing the shock on the drive, which causes the clutch to no longer slip and all of the engine's torque is once again transmitted to the stern drive.
The invention is designed such that the torque required to cause the clutch to slip is nearly identical to the torque that the clutch will transmit while slipping. Essentially, the invention will always transmit the engine's torque to the drive even while protecting the drive from overloading. This unique aspect continues to apply regulated torque to the drive and will make it virtually impossible for the operator to detect the invention's operation.
Applicant's invention minimizes the possibility of the stern drive being overloaded due to the forces exerted on the drive when the propeller leaves and then re-enters the water or when subjected to impact with a submerged object. As propeller technology advances and X-dimensions are raised in search of more speed the stress on the stern drive increases dramatically. Many boaters are seeing huge gains in efficiency at cruising speeds when switching to five, six and even seven blade propellers. This gain in efficiency will instantly overload a drive when a surfacing propeller re-enters the water. The invention allows these boaters to benefit from the increased efficiency without sacrificing reliability. Furthermore, the invention can help protect the drive train from catastrophic damage should the propeller strike a submerged object, thus avoiding costly repairs.
The invention can be used on most all watercraft having a stern drive, even V hulls or catamarans with single or multiple engines. Torque values for the invention can be adjusted in the field to allow the user to match a given engine output. Obviously, if the torque ratings are adjusted to exceed the drive's rating then the advantages are reduced. Accurate engine data is required to ensure the clutch does not slip under normal conditions which would lead to excessive wear and a limited life span.